During an engine start, the engine airflow is zero, and the intake manifold pressure is at ambient pressure. As a result, a significant amount of fuel may be added at an engine start to maintain stoichiometry of exhaust gases (which is required for emissions control). This leads to a large initial engine torque output which can cause the engine speed to flare up quickly until the intake manifold pressure and the engine speed are regulated. The engine speed flare-up at the engine start can result in a higher fuel consumption, degrading fuel economy. In addition, the flare-up results in NVH issues including extra noise from the engine, and palpable driveline vibrations.
Various approaches have been developed for reducing the speed flare at the engine start. For example, as shown by Ma et al. in U.S. Pat. No. 8,694,231, spark timing adjustments (in particular, spark timing retard) may be used to control the engine speed at the engine start towards a target engine speed that reduces flare. In another example, as shown by Marentette in U.S. Pat. No. 7,281,512, an assembly including a deformable member and a resilient member is coupled inside an intake plenum and used to vary the volume of the intake manifold. At the low engine speeds of an engine start, the assembly reduces the volume within the plenum, thereby reducing engine flare.
However the inventors herein have identified potential issues with such approaches. As one example, the reliance on spark timing retard reduces the fuel economy of the engine. As another example, the reliance on a dedicated assembly for selectively varying the volume of the intake manifold adds component and control costs.
The inventors herein have recognized that the compressor of an electric supercharger in staged boosted engine system can be advantageously used to reduce engine start-up speed flare. As such, engine systems may be configured with boosting devices, such as turbochargers or superchargers, for providing a boosted aircharge and improving peak power outputs. Therein the use of a compressor allows a smaller displacement engine to provide as much power as a larger displacement engine, but with additional fuel economy benefits. Further, one or more intake charging devices may be staged in series or parallel to an intake turbocharger to improve turbocharged engine boost response. In such an engine system, the compressor of an electric supercharger may be selectively operated in a reverse direction during the engine start-up to generate intake manifold vacuum. The resulting lower intake manifold pressure reduces the engine start fuel requirement, producing lowering engine torque, and reducing engine flare. One example method for reducing engine flare includes: during an engine start, spinning a compressor backwards to lower intake manifold pressure, a speed of the compressor based on engine speed. In this way, a quality of engine starts is improved.
As one example, a boosted engine system may include an electric supercharger coupled upstream of a turbocharger. In response to an engine start, an electric motor coupled to the controller of the electric supercharger may be operated to spin the supercharger compressor backwards (that is, in a direction opposite to the direction the compressor is spun in for boosted charge delivery). For example, the electric motor may be coupled to a reversing circuit that enables the supercharger compressor to be spun backwards. Alternatively, the motor may be coupled to an H-bridge that enables the compressor to be spun in forward and reverse directions. The spinning may be performed at a speed and for a duration that enables manifold pressure to be lowered to a threshold pressure. As a result of the lower pressure, less air needs to be pumped into the engine cylinders during the start (since the cylinder charge is a function of the intake pressure). Engine fueling may then be scheduled based on the lower charge requirement.
Herein, the lower intake air pressure results in a lower cylinder fueling requirement, and consequently a reduction in the engine torque output, and thereby engine speed flare. The reverse rotation of the supercharger compressor may be continued until an engine idling speed is reached. Once the engine has reached engine idle speed, backward rotation of the supercharger compressor may be ended. Optionally thereafter, forward rotation of the supercharger compressor may be enabled for boost control (such as to reduce turbo lag while the downstream turbocharger spins up).
In this way, by generating vacuum in an engine intake manifold during an engine start, the intake manifold pressure at the engine start may be lowered. The technical effect of rotating an electric supercharger compressor in reverse to generate vacuum is that existing engine components can be advantageously used to generate the vacuum, providing component and cost reduction benefits. The technical effect of using vacuum to lower an intake manifold pressure at an engine start is that less air needs to be pumped into engine cylinders, lowering the cylinder's fueling requirement. By delivering a smaller amount of fuel to an engine cylinder based on a lowered cylinder charge requirement, the engine may be operated at stoichiometry while an engine speed flare at the engine start-up is reduced. By lowering the engine torque and speed flare at the engine start, engine start NVH issues are reduced while fuel economy is improved. Overall, the quality and repeatability of engine starts are improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.